Monday, August 6, 2007

PS 12-141: Rate of change and spatial pattern of woody and herbaceous vegetation in a semi-arid savanna

Ana V. González and Norma L. Fowler. University of Texas at Austin

As woody encroachment turns central Texas savannas into woodland, there is a concomitant change in the spatial configuration of the herbaceous and woody vegetation. Our goals are (1) to quantify the rate and magnitude of the increase in woody cover, (2) to quantify the relationship between cover and spatial configuration at a given date across the landscape, and, in particular, (3) to determine to what extent the relationship between cover and each individual spatial configuration metric is the same for woody and herbaceous vegetation. Historical aerial photographs of Pedernales Falls State Park from 1951, 1965, 1981, 1995 and 2004 were processed into binary images representing woody and herbaceous vegetation. Cover and spatial configuration metrics of each vegetation type were calculated for randomly-located plots of four sizes using FRAGSTATS. Mean woody cover increased at an average rate of 1.1% per year from 1951 to 2004. As expected, plots with more woody cover had, on average, fewer and larger patches with shorter distances between neighboring patches, and the herbaceous vegetation had the opposite trend. Many of the metrics of spatial configuration were well-predicted by simple functions of cover. However, the relationship between a given metric of herbaceous spatial configuration and herbaceous cover often was different from the relationship between the same metric of woody spatial configuration and woody cover. Further, in many instances, the shape of the function describing the relationship between cover and a given metric differed among years and/or among plot sizes.

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